Traditionally, telephony communications within the United States were handled by the public switched telecommunications network (PSTN). The PSTN can be characterized as a network designed for voice communications, primarily on a circuit-switched basis, with full interconnection among individual networks. The PSTN network is largely analog at the local loop level, digital at the backbone level, and generally provisioned on a wireline, rather than a wireless, basis. The PSTN includes switches that route communications between end users. Circuit switches are the devices that establish connectivity between circuits through an internal switching matrix. Circuit switches set connections between circuits through the establishment of a talk path or transmission path. The connection and the associated bandwidth are provided temporarily, continuously, and exclusively for the duration of the session, or call. While developed to support voice communications, circuit switches can support any form of information transfer (e.g., data and video communications).
In a traditional PSTN environment, circuit switches include central office (CO) exchanges, tandem exchanges, access tandem exchanges, and international gateway facilities. Central offices, also known as exchanges, provide local access services to end users via local loop connections within a relatively small area of geography known as an exchange area. In other words, the CO provides the ability for a subscriber within that neighborhood to connect to another subscriber within that neighborhood. Central offices, also known as end offices, reside at the terminal ends of the network. In other words, COs are the first point of entry into the PSTN and the last point of exit. They are also known as class 5 offices, the lowest class in the switching hierarchy. A class 5 telephone switch communicates with an analog telephone using the analog telephony signals in the well-known analog format. The class 5 telephone switch provides power to the telephone; detects off-hook status of the telephone and provides a dial tone in response; detects dual-tone multi-frequency signals from the caller and initiates a call in the network; plays a ringback tone to the caller when the far-end telephone is ringing; plays a busy tone to the caller when the far-end telephone is busy; provides ring current to the telephone on incoming calls; and provides traditional telephone services such as call waiting, call forwarding, caller ID, etc.
In an effort to increase the amount and speed of information transmitted across networks, the telecommunications industry is shifting toward broadband packet networks which are designed to carry a variety of services such as voice, data, and video. For example, asynchronous transfer mode (ATM) networks have been developed to provide broadband transport and switching capability between local area networks (LANs) and wide area networks (WANs). The Sprint ION network is a broadband network that is capable of delivering a variety of services such as voice, data, and video to an end user at a residential or business location. The Sprint ION network has a wide area IP/ATM or ATM backbone that is connected to a plurality of local loops via multiplexors. Each local loop carrier's ATM over ADSL (asymmetric digital subscriber line) traffic to a plurality of integrated service hubs (ISHs), which may be at either residential or business locations.
An ISH is a hardware component that links business or residential user devices such as telephones and computers to the broadband, wide area network through a plurality of user interfaces and at least one network interface. A suitable ISH is described in co-pending U.S. patent application Ser. No. 09/226,575 entitled "Multi-Services Communications Device," filed on Jan. 7, 1999 (Sprint docket number 1246), which is incorporated by reference herein in its entirety. The network interface typically is a broad band network interface such as ADSL, T1, or HDSL-2. Examples of user interfaces include telephone interfaces such as plain old telephone system (POTS) ports for connecting telephones, fax machines, modems, and the like to the ISH; computer interfaces such as ethernet ports for connecting computers and local area networks to the ISH; and video ports such as RCA jacks for connecting video players, recorders, monitors, and the like to the ISH.
In providing telephony services over a broadband network, the ISH connects a telephone in the customer's premises to a network element such as a service manager. This connection between the telephone and the network element is typically an ATM connection, which is much different than the traditional analog line to the local switch. ATM connections usually do not support analog telephony signals, such as off-hook, dial tone, and busy signals. Therefore, the ISH must provide many of the telephony functions traditionally provided by the telephone provider central office such as detect off-hook conditions, on-hook connections, and digits as well as provide the telephones with dial tone, ring current, ringback, and busy signals. The terms off-hook and off-hook condition as used herein are generic terms meaning that a user device (whether telephone, facsimile machine, modem, etc.) connected to a telephone line is attempting to access and use the line.
The numerous functions which must be performed by the ISH are handled by a number of different circuits physically located on a number of circuit boards plugged into a backplane. A backplane is itself a printed circuit board with a number of sockets for receiving the other circuit boards which make up the ISH. One board carries a power converter which receives AC power from the local power grid and provides a DC voltage, typically 12 volts, to the backplane. Each of the other cards plugged in to the backplane receives this DC voltage and uses it to power circuitry carried on the boards. Most of the circuits used in telecommunication applications require lower voltages to operate, typically 5 volts and 3.3 volts. Although these other voltages may be provided by the power supply and connected to the backplane for connection to the other cards, a number of problems have been recognized with this approach. The additional power supply rails in the backplane use more of the available spaces in the backplane sockets, increase resistive losses and increase system noise. The lower voltages require close regulation. Each of the low voltage supplies must be overdesigned since it is not necessarily known in advance how much power the functional cards will require.
Use of a distributed power arrangement avoids these problems. In a distributed power system, the main power supply provides only one relatively high voltage level, typically 12 to 48 volts, to the backplane. The lower voltages are provided by power converters on each card. This helps reduce system noise by isolating functional blocks and allows for failure isolation. Each converter can be optimally sized for the functional circuitry on its own circuit board. The main power supply need not be closely regulated, since the distributed converters provide control on each board.
Power converters located on the functional cards, however, cause a problem with hot swapping, i.e. plugging in cards while the power is on. This is a particular concern in equipment intended for installation on customer premises, because the customer may add or replace boards without first turning off the power to the system.